Many multiple access communication techniques are known including time division multiple access (TDMA), frequency division multiple access (FDMA), orthogonal and vector orthogonal frequency division multiplexing (OFDM or VOFDM), code division multiple access (CDMA), hybrids such as GSM, etc. which allow a single resource, should as a radio channel, to be shared amongst multiple users. One common use for multiple access systems with radio channels is mobile telephone systems wherein multiple handsets share the radio resources of a base station.
The ability of a radio receiver employing multiple access techniques to correctly receive a signal transmitted to it is generally limited by the signal to noise ratio (SNR) the receiver experiences. The SNR experienced at a receiver is the ratio of the desired received signal to all other noise sources, including thermal noise, radio noise (noise from electrical devices such as motors, etc.), transmissions from adjacent transmitters (such as adjacent cells or sectors in a mobile telephone system) and other, non-orthogonal, signals transmitted from the transmitter to which the receiver is listening.
As used herein the term “orthogonal signal” is intended to include all signals which are arranged at transmission to have cross correlations that are ideally zero, or very small, e.g. CDMA signals are made orthogonal via application of Walsh Codes, TDMA signals are made orthogonal via assignment of time slots, etc. It should be noted that an orthogonal signal can be received at a receiver with its orthogonality somewhat reduced, due to multipath and other effects.
Clearly, the better the SNR experienced at a receiver, the better the ability of the receiver to correctly receive the signal and the better the theoretical capacity of the system, as will be discussed further below.
One example of a widely used multiple access technique is code division multiple access (CDMA), and specifically the direct sequence implementation of CDMA, which has recently gained significant support as the multiple access technique of choice for advanced wireless communication systems, such as mobile telephones or wireless local loop systems. As is known, CDMA can offer advantages over many other multiple access techniques, in that planning and management of the network is generally simplified, with the guard bands or guard times typically required in FDMA or TDMA systems, for example, not being required and good frequency reuse being obtained relatively easily.
As mentioned above, increases to the SNR experienced at a CDMA receiver are advantageous, Specifically, as the SNR experienced by a CDMA receiver is increased, more efficient use can be made of the CDMA code space, with modulation orders being increased (for example from QPSK to QAM 16) and/or higher rate error correcting codes can be used (for example increasing the code rate from ⅓ rate to ⅔ rate). As CDMA code space is a limiting factor in the capacity of a CDMA communications system, it is always desired to make efficient use of the code space.
Further advantages are obtained when transmissions in CDMA are performed at the lowest power level which is sufficient to provide the minimum SNR required for reception of the signal at the receiver at acceptable error rates. By broadcasting to a first receiver at this minimum power level, or very close to it, the interference (noise) experienced at other receivers can be reduced, further increasing the efficiency and capacity of the CDMA system as the SNR's of those other receivers will be improved.
Other multiple access systems benefit from improved SNR's in manners similar to those of CDMA and, generally, an increase in the SNR of signals received at a receiver results in improved capacity and/or reliability of the communications system.
Accordingly, it is desired to have a system, method and apparatus which can allow a multiple access communications receiver to improve the SNR of desired signals it otherwise receives from a transmitter, thus providing for overall improved performance of the system.